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A module that is everything required to understands Druid Segments
There is a newer version: 30.0.1
/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */


package org.apache.druid.segment;

import it.unimi.dsi.fastutil.objects.Object2IntMap;
import it.unimi.dsi.fastutil.objects.Object2IntOpenHashMap;

import javax.annotation.Nullable;
import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.concurrent.atomic.AtomicLong;

/**
 * {@link Comparator} based {@link DimensionDictionary}
 *
 * there are a lot of unused methods in here for now since the only thing this is used for is to build up
 * the unsorted dictionary and then it is converted to a {@link ComparatorSortedDimensionDictionary}, but
 * leaving the unused methods in place for now to be basically compatible with the other implementation.
 *
 * This version is not thread-safe since the only current user doesn't use the dictionary for realtime queries, if this
 * changes we need to add a 'ConcurrentComparatorDimensionDictionary'.
 */
public class ComparatorDimensionDictionary
{
  public static final int ABSENT_VALUE_ID = -1;

  @Nullable
  private T minValue = null;
  @Nullable
  private T maxValue = null;
  private volatile int idForNull = ABSENT_VALUE_ID;

  private final AtomicLong sizeInBytes = new AtomicLong(0L);
  private final Object2IntMap valueToId = new Object2IntOpenHashMap<>();

  private final List idToValue = new ArrayList<>();
  private final Comparator comparator;

  public ComparatorDimensionDictionary(Comparator comparator)
  {
    this.comparator = comparator;
    valueToId.defaultReturnValue(ABSENT_VALUE_ID);
  }

  public int getId(@Nullable T value)
  {
    if (value == null) {
      return idForNull;
    }
    return valueToId.getInt(value);
  }

  @SuppressWarnings("unused")
  @Nullable
  public T getValue(int id)
  {
    if (id == idForNull) {
      return null;
    }
    return idToValue.get(id);
  }

  public int size()
  {
    // using idToValue rather than valueToId because the valueToId doesn't account null value, if it is present.
    return idToValue.size();
  }

  /**
   * Gets the current size of this dictionary in bytes.
   *
   * @throws IllegalStateException if size computation is disabled.
   */
  public long sizeInBytes()
  {
    return sizeInBytes.get();
  }

  public int add(@Nullable T originalValue)
  {
    if (originalValue == null) {
      if (idForNull == ABSENT_VALUE_ID) {
        idForNull = idToValue.size();
        idToValue.add(null);
      }
      return idForNull;
    }
    int prev = valueToId.getInt(originalValue);
    if (prev >= 0) {
      return prev;
    }
    final int index = idToValue.size();
    valueToId.put(originalValue, index);
    idToValue.add(originalValue);

    // Add size of new dim value and 2 references (valueToId and idToValue)
    sizeInBytes.addAndGet(estimateSizeOfValue(originalValue) + (2L * Long.BYTES));

    minValue = minValue == null || comparator.compare(minValue, originalValue) > 0 ? originalValue : minValue;
    maxValue = maxValue == null || comparator.compare(maxValue, originalValue) < 0 ? originalValue : maxValue;
    return index;
  }

  @SuppressWarnings("unused")
  public T getMinValue()
  {
    return minValue;
  }

  @SuppressWarnings("unused")
  public T getMaxValue()
  {
    return maxValue;
  }

  @SuppressWarnings("unused")
  public int getIdForNull()
  {
    return idForNull;
  }

  public ComparatorSortedDimensionDictionary sort()
  {
    return new ComparatorSortedDimensionDictionary(idToValue, comparator, idToValue.size());
  }

  /**
   * Estimates the size of the dimension value in bytes. This method is called
   * only when a new dimension value is being added to the lookup.
   *
   * @throws UnsupportedOperationException Implementations that want to estimate
   *                                       memory must override this method.
   */
  public long estimateSizeOfValue(T value)
  {
    throw new UnsupportedOperationException();
  }
}